The present invention relates to a digital signal demodulation system, in particular, relates to such a system which is free from waveform distortion and/or cross polarization interference by fading.
We have first considered a digital signal demodulation system with a cross polarization interference canceller as shown in FIG. 1. In the figure, each of main signal which suffers from interference, and an interference signal is demodulated by each of demodulators, DEM(A), and DEM(B), respectively. The demodulated signal is applied to an analog-digital converter (A/D(A), A/D(B)), which converts an input analog signal to digital form through sampling of an input signal using the clock signal regenerated in each branch. A transversal filter is used to cancel the interference component in the main signal. The output of the analog-digital converter A/D(B) in the interference branch is applied to the transversal filter, the output of which is applied to the combiner, which also receives the output of the analog-digital converter A/D(A) in the main branch. Thus, the interference component in the main branch is cancelled in the combiner. The tap coefficient of the transversal filter is controlled by the Control using a zero-forcing (ZF) method which provides the tap coefficient by obtaining the decision bit which shows a polarity of a signal, and an error signal; multiplying polarity of the decision signal and the polarity of the error signal; and integrating the product.
However, the configuration of FIG. 1 has the following disadvantages.
(a) When an analog input signal is higher than the allowable upper level of an analog-digital converter, the A/D converter saturates. This condition occurs when an input signal is distorted by waveform distortion and/or interference. The saturation of the A/D converter would essentially provide non-linear distortion, and deteriorate the compensation ability of a transversal filter.
(b) When a signal of a second polarization branch does not synchronize with the signal of a first polarization branch, the operation in the transversal filter and the combiner is asynchronous, and in that condition, the cancellation of an interference component is impossible.
(c) A prior ZF (zero-forcing) method for providing a tap coefficient of a transversal filter does not operate well when the inter-symbol interference is large, and further, the pull-in range for convergence becomes narrow when it becomes out of control, and deteriorates the characteristics of the demodulation system itself.